story identification - Post-apocalyptic novel: an astronaut returns to an Earth reverted to the stone age

I was also looking for this great book and just found it and your post during my search. If you have not found it, The Return by Richard Maynard.

‘The Return’ starts with a voyage into interstellar space by a team of seven British astronauts. Their goal is to travel faster-than-light to the Alpha Centauri system and back, a journey which should take the equivalent of sixteen years of Earth-time. Unfortunately the ship encounters navigational problems and goes off-course; by the time the crew corrects the error, they have been traveling for 15 years ship-time. The crew must confront the awful fact that perhaps as many as 60 years...or as many as 400 years.... have passed on Earth since their departure.

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And the humans that populate these landscapes are semi-literate savages wrapped in animal furs and toting spears. And they are not friendly, as the starship crew soon discovers….